For a hybrid vehicle, for example, in addition to an existing brake system for generating a brake force through a hydraulic circuit, a by-wire brake system for generating a brake force through an electric circuit is adopted. In such a by-wire brake system, the operation amount of a brake pedal by a driver is converted into an electrical signal to be provided to an electric motor for driving a piston of a slave cylinder (hereinafter, referred to as an “electric servo brake system”). Then, a hydraulic brake pressure is generated by compressive driving of the piston by the electric motor to be applied to the electric servo brake system. Thus generated hydraulic brake pressure operates wheel cylinders to generate brake forces caused by friction.
In addition to the control of the friction braking caused by an electric servo brake system, the control of the regenerative braking caused by a wheel driving electric motor is performed in such a by-wire brake system. A wheel driving electric motor is used as an electric power generator in the control of the regenerative braking caused by a wheel driving electric motor, wherein a brake force is generated by a regenerative torque generated by the wheel driving electric motor.
Further, conventionally, in order to obtain a brake force corresponding to the operation amount of a brake pedal by a driver, a technology is known that performs control integrating the control of regenerative braking caused by a wheel driving electric motor and the control of friction braking caused by an electric servo brake system. The recovery efficiency of kinetic energy can be increased by this integrated brake control technology.
Among by-wire brake systems to which the above-described integrated brake control technology is applied, known is one provided with an ABS (Antilock Brake System) having a function to avoid a locked state of wheels in braking a vehicle (see Patent Literature 1). In the brake force control technology according to Patent Literature 1, the behavior of the vehicle is estimated, based on a signal from a wheel speed sensor, an acceleration sensor, and the like, and based on a result of this estimation, brake forces applied to the wheels are controlled so as to stabilize the behavior of the vehicle.
In detail, in the brake force control technology according to Patent Literature 1, if the result of a diagnosis of the ABS is normal, it is determined whether or not ABS control is to be executed based on the slip ratios of the wheels, and torque control of a wheel driving electric motor is performed, based on the operational state of the ABS according to this result of determination. Concretely, an electric motor for driving wheels is controlled to be in a power running state when ABS control is performed, and is controlled to be in a regenerative state when ABS control is not performed.
On the other hand, if the diagnosis result of the ABS is abnormal, the wheel driving electric motor is immediately prohibited from being controlled in the regenerative brake control state and is controlled to be in the power running state.
In brief, in the brake force control technology according to Patent Literature 1, if a diagnosis result of the ABS is abnormal, the wheel driving electric motor is subjected to drive control so as to avoid locking of the wheels caused by erroneous regenerative braking.
Thus, in the brake force control technology according to Patent Literature 1, the wheel driving electric motor can be appropriately controlled to stabilize the behavior of the vehicle.